A thermoelectric device is a device that adjusts a temperature of a component arranged on the thermoelectric device by current flowing through the thermoelectric device. Such a thermoelectric device can be manufactured in a very small size. Since mechanical properties such as compression or expansion of gas are not changed when the component is cooled and heated, the thermoelectric device is widely adopted to adjust a temperature of a small component. For example, in an optical communication field, a wavelength of a semiconductor laser used for optical communication depends on a temperature of the semiconductor laser.
In recent years, in a DWDM (Dense Wavelength Division Multiplexing) scheme used for optical communication, light having a frequency interval of 100 GHz is used, and such a frequency interval has a wavelength interval of about 0.8 nm at a wavelength band of 1.55 μm. In contrast, in a DFB-LD (Distributed Feedback Laser Diode) used for optical communication, when a temperature of a semiconductor laser diode chip is changed by about 1° C., a laser light beam released from the semiconductor laser is changed by about 0.1 nm. Accordingly, in general, when a temperature of the DFB-LD is changed by about 8° C., a wavelength of light released from the laser corresponds to another channel of the DWDM to cause communication crosstalk. Therefore, in the DWDM, the temperature of the semiconductor laser needs to be very precisely adjusted.
The thermoelectric device has been successfully used to control the temperature of the DFB-LD chip. In order to control a temperature above the thermoelectric device by using the thermoelectric device, it is required that a device called a thermistor is first mounted on the thermoelectric device and a temperature of the thermoelectric device is measured.
FIG. 1 is a perspective view of a conventional thermistor. A conventional thermistor 100 is a device having characteristics in which a resistance value is varied depending on a temperature, and typically has a rectangular shape whose bottom is wide. A metal thin film is coated on top and bottom surfaces of the thermistor 100 to be able to be electrically connected to an external electrode. The thermistor 100 is a device that measures a temperature of a bottom on which the thermistor is mounted. The thermistor is configured to measure a temperature of the thermistor 100 by connecting the metal thin film deposited on the top and bottom surfaces of the thermistor 100 of FIG. 1 to the external electrode and measuring a resistance of the thermistor 100 which is varied depending on a temperature.
FIG. 2 shows a method for connecting the thermistor having the shape shown in FIG. 1 to the external electrode. Since the thermistor 100 having the shape shown in FIG. 1 needs to allow the bottom surface on which the metal thin film is deposited to be attached to a substrate to be measured, the bottom surface of the thermistor 100 having the shape shown in FIG. 1 is attached onto a sub-mount 600 in which metal is deposited on a top surface by using a material having electrical conductivity, and then one metal electric wire 310 is extracted from the top surface of the thermistor 100 to be electrically connected to the external electrode. Further, the other metal electric wire 320 is extracted from the top surface of the sub-mount 600 that is electrically connected to the bottom surface of the thermistor 100 to be electrically connected to the external electrode. By doing this, the resistance of the thermistor 100 is measured. In this way, FIG. 2 illustrates that two electric wires 310 and 320 are extracted from the top surface of the thermistor 100 and the top surface of the sub-mount 600 that is electrically connected to the thermistor 100.
FIG. 3 is an internal configuration diagram of a laser diode package and illustrates a state in which the thermistor is attached to a TO (Transistor Outline) type laser diode package including a thermoelectric device in order to measure a temperature of a top surface of the thermoelectric device. In the laser diode package, the thermistor 100 is disposed on a thermoelectric device 200, the top surface of the thermistor 100 is connected to an electrode pin 410 through the metal electric wire 310, and the top surface of the sub-mount 600 onto which the thermistor 100 is attached is connected to an electrode pin 420 through the metal electric wire 320. By doing this, a lower electrode of the thermistor 100 is connected to the external electrode. Reference numeral 400 denotes the electrode pins protruding downward through the laser diode package.
In the shape shown in FIG. 3, temperatures of the electrode pins 410 and 420 of the laser diode package are not adjusted, and only a temperature of the top surface of the thermoelectric device 200 is adjusted by a direction and magnitude of current flowing in the thermoelectric device 200. Accordingly, when an external temperature of the laser diode package is changed, the temperatures of the electrode pins 410 and 420 of the laser diode package are changed depending on the external temperature.
FIG. 4 is a cross-sectional view showing a state in which the laser diode package of FIG. 3 is covered by a package lid to complete the package. In FIG. 4, in order to accurately control the temperature of the top surface of the thermoelectric device 200 by using the thermistor 100 and the thermoelectric device 200, the thermistor 100 needs to accurately measure the temperature of the top surface of the thermoelectric device 200. However, as shown in FIG. 3, the thermistor 100 is directly connected to the electrode pin 410 having a temperature which is approximate to an external environment temperature and is different from the temperature of the top surface of the thermoelectric device 200 through the metal electric wire 310, so that an error occurs in measuring the temperature of the top surface of the thermoelectric device 200 by the thermistor 100. Such a phenomenon causes a severe error when the thermistor 100 has a very small size and a length of the metal electric wire 310 between the electrode pin 410 and the thermistor 100 is short. Further, since a temperature of a package lid 700 of FIG. 4 is also the same as the external environment temperature, an error is caused in measuring the temperature of the thermistor 100 due to thermal convection and thermal radiation from the package lid 700.
As shown in FIGS. 3 and 4, when the conventional TO type laser diode package is manufactured such that the top surface of the thermistor 100 is directly connected to the electrode pin 410 and the thermistor 100 is exposed to internal gas of the package, a measurement error of about 0.04° C. is caused in the thermistor 100 when the external environment temperature is changed by 1° C. Such a measurement error means that the temperature of the top surface of the internal thermoelectric device 200 is changed by about 5° C. when the external environment temperature is changed by about 125° C., and the measurement error of such a degree becomes a factor by which it is difficult to precisely adjust a wavelength of a laser diode to an extent capable of being applied to a DWDM.